Semiconductor lasers and processes for the planarization of semiconductor lasers

ABSTRACT

A laser structure may include a substrate, an active region arranged on the substrate, and a waveguide arranged on the active region. The waveguide may include a first surface and a second surface that join to form a first angle relative to the active region. A material may be deposited on the first surface and the second surface of the waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication No. 62/339,581, filed on May 20, 2016, the contents of whichis incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to semiconductors, and moreparticularly, to semiconductor lasers and processes for theplanarization of semiconductor lasers.

BACKGROUND OF THE DISCLOSURE

Semiconductor lasers are typically realized on a wafer by epitaxiallydepositing multiple layers of semiconductor materials such as InP, GaAs,GaN and ternary or quaternary semiconductor material on a wafer usingMetalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy(MBE) deposition on a semiconductor substrate surface.

Subsequently, multiple semiconductor fabrication steps are applied tothe wafer to realize a laser optical cavity which contains a deviceactive region with facets and metallic electrical contacts. Typically,facets are formed by cleaving or etching the semiconductor material. Theapplication of an electrical potential allows for electrical currentflow through the active region of the device which causes photons to beemitted out of the facets.

During the operation of semiconductor laser diodes, a large amount ofheat is generated within the active region of the device. Generally, theperformance of laser diodes is better when the active region is at alower temperature. Most laser diodes are mounted active-side-up wherethe substrate is in contact with a thermally conductive material such asan AlN submount. Alternatively, laser diodes with appropriate designsmay be mounted active-side-down to reduce a thermal path between theactive region and the submount.

Lasers play a large role in optical communication systems. Some of thetransmission speeds are at or above 25 Gbits per second and the lasersare typically required to operate at temperatures above 50° C.Typically, the hotter the active region, the worse the high speedperformance of the semiconductor laser. As such, heat spreading and heatremoval from the active region remains a significant challenge inachieving high speed operations, especially at elevated temperatures.

At high frequencies of operation, laser capacitance can impact theperformance that can be obtained from a semiconductor laser. As such,reduction of capacitance associated with the laser at high frequenciesmay be necessary to allow appropriate performance of the laser.

A Buried-Heterostructure (BH) approach can be used for spreading heataway from the active region of a laser. Such an approach typicallyinvolves etching through the active region and regrowing a high thermalconductivity thin film such as InP using either MOCVD or MBE to create agood thermal flow path from the active region to a device metal layer.However, there are some disadvantages of this approach. Mosthigh-performance laser active regions incorporate Al. Etching through anactive region with Al content typically results in the formation ofaluminum oxide (AlO₂) on sidewalls of the active layer. Aluminum oxideis hard to remove prior to regrowth required in the formation of a BHlaser. The presence of aluminum oxide may result in defects insubsequent layers during the regrowth process that may degrade deviceperformance. Furthermore, subsequent regrowth adds cost. The followingpublications highlight various approaches to address such challenges:“Improved Regrowth Interface of AlGaInAs/InP-Buried-HeterostructureLasers by In-Situ Thermal Cleaning” by Takino, et al., IEEE Journal ofQuantum Electronics, Volume. 48, Number 8, pages 971-979, August 2012;“Theoretical study of the temperature dependence of 1.3 μm AlGaInAs—InPmultiple-quantum-well lasers” by Pan, et al., IEEE Journal of QuantumElectronics, Volume 32, Number 12, pages 2133-2138; December 1996 andU.S. Pat. No. 6,821,801 B1.

In view of the foregoing, it can be appreciated that there may be a needfor a high-performance laser that does not rely on expensive BHtechnology.

SUMMARY OF THE DISCLOSURE

In some embodiments, a laser structure may include a substrate, anactive region arranged on the substrate, and a waveguide arranged on theactive region. The waveguide may include a first surface and a secondsurface that join to form a first angle relative to the active region.The laser structure may further include a material deposited on thefirst surface and the second surface of the waveguide.

In some embodiments, the first angle may be less than ninety degrees.

In some embodiments, the waveguide may further include a third surfaceand a fourth surface that join to form a second angle relative to theactive region. The material may be deposited on the third surface andthe fourth surface.

In some embodiments, the second angle may be less than ninety degrees.

In some embodiments, the material may be one of MgO, MgF₂, SiO₂, orSi₃N₄.

In some embodiments, the material may have a dielectric constant below10 in a frequency range up to 50 GHz.

In some embodiments, the material may be non-conducting.

In some embodiments, the waveguide may include a fifth surface arrangedbetween the first surface and the third surface, and the laser structuremay further include a first contact arranged on the fifth surface and asecond contact arranged on the substrate. The first contact may beconfigured to bias the laser structure by delivering electrical currentto the laser structure.

In some embodiments, the laser structure may further include at leastone facet.

In some embodiments, the at least one facet may be formed in the activeregion.

In some embodiments, a laser structure may include a substrate, anactive region arranged on the substrate, and a waveguide arranged on theactive region. The waveguide may include a first ridge and a secondridge. The first ridge may form a first air gap and the second ridge mayform a second air gap. The laser structure may further include a metallayer arranged as a bridge over the first and second air gaps.

In some embodiments, the first ridge may form a first angle of less thanninety degrees relative to the active region and the second ridge mayform a second angle of less than ninety degrees relative to the activeregion.

In some embodiments, a first contact may be arranged on a surface of thewaveguide and the first contact may be configured to bias the laserstructure by delivering electrical current to the laser structure.

In some embodiments, the laser structure may further include at leastone facet.

In some embodiments, the at least one facet may be formed in the activeregion.

In some embodiments, a method of fabricating a laser structure mayinclude arranging an active region on a substrate and arranging awaveguide on the active region. The waveguide may include a first ridgeand a second ridge. The method may further include depositing a polymeron the waveguide such that the polymer coats underneath the first ridgeand second ridge, depositing at least one layer of resist on thepolymer, depositing a metal layer on the at least one layer of resist,and removing the deposited polymer and the deposited at least one layerof resist.

In some embodiments, the first ridge may form a first angle of less thanninety degrees relative to the active region and the second ridge mayform a second angle of less than ninety degrees relative to the activeregion.

In some embodiments, the method may further include arranging a firstcontact on a surface of the waveguide. The first contact may beconfigured to bias the laser structure by delivering electrical currentto the laser structure.

In some embodiments, the method may further include forming at least onefacet in the active region.

In some embodiments, the at least one facet may be formed by etching.

In some embodiments, a fixture for electron beam evaporation may includea wafer plate configured to support a laser structure and an integratedheater configured to emit heat at an evaporation temperature. The heatemitted by the integrated heater may be adjustable and an angle of thefixture may be adjustable.

In some embodiments, the wafer plate may be further configured to orientthe laser structure relative to an evaporant.

In some embodiments, the orientation of the laser structure may subjectat least one ridge of the laser structure to the evaporant.

In some embodiments, the orientation of the laser structure may bevaried based on an angle of at least one ridge of the laser structure.

The present disclosure will now be described in more detail withreference to particular embodiments thereof as shown in the accompanyingdrawings. While the present disclosure is described below with referenceto particular embodiments, it should be understood that the presentdisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein, and with respect to which the present disclosure maybe of significant utility.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present disclosure,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the present disclosure, but are intended to beillustrative only.

FIG. 1 shows a cross section of a semiconductor laser in accordance withembodiments of the present disclosure.

FIG. 2 shows a cross section of another semiconductor laser inaccordance with embodiments of the present disclosure.

FIG. 3 shows a cross section of another semiconductor laser inaccordance with embodiments of the present disclosure.

FIG. 4 shows a cross section of another semiconductor laser inaccordance with embodiments of the present disclosure.

FIG. 5 shows top side views of an adjustable fixture configured tosupport a semiconductor structure in accordance with embodiments of thepresent disclosure.

FIG. 6A shows an adjustable fixture in accordance with embodiments ofthe present disclosure.

FIG. 6B shows an adjustable fixture in accordance with embodiments ofthe present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure and the related advantages are described andhighlighted in the following description and accompanying figures whichare not necessarily drawn to scale. Detailed descriptions of structureand processing techniques are omitted so as to not unnecessarily obscurethe present disclosure.

FIG. 1 shows a cross section of a semiconductor laser 100. Laser 100 isa dovetail ridge waveguide laser structure with at least one electricalcontact. Laser 100 may include a bottom layer 102. Bottom layer 102 maybe a metal layer, and may provide an electrical contact layer adjacentto layer 104. Layer 104 may be an N-type semiconductor, and may belocated between layers 102 and active region 106. Layer 104 may be anInP substrate, for example. Active region 106 may provide for theemission of photons from laser 100. Layer 108 may be located adjacent toactive region 106. Layer 108 may be a P-type semiconductor. Layer 108may include a dovetail ridge waveguide (RWG) 116. Layer 110 may belocated adjacent to layer 108. Layer 110 may be an insulation layer. Forexample, layer 110 may be an insulation layer of SiO₂ deposited on layer108 to cover exposed surfaces of layer 108. On the top of dovetail RWG116, an opening 118 in layer 110 may be formed. A top metal layer 112may be deposited over layer 110 and over the opening in layer 110 ondovetail RWG 116. Top metal layer 112 may therefore provide anelectrical contact layer for biasing the device on the p-side via theopening in layer 110 on dovetail RWG 116. A bond pad 114 may be attachedto top metal layer 112. Chemically assisted ion beam etch (CAIBE) may beused to form etched facets on a face of laser 100 defined by the X and Yaxes of laser 100, shown in FIG. 1. The facets may be etched in activelayer 106. Cleaving could also be used to form the etched facets.

Laser 100 may provide a high degree of topology. Laser 100, however, mayinclude reduced p-side metal coverage and a reduced thermal conductivitypath from active region 106. Dovetail RWG 116 may form a smaller ridgewidth relative to other ridge widths. The ridge width of dovetail RWG116 may be configured to confine light, but may also be configured toallow a larger area for a p-side metal contact to reduce contactresistance.

Dovetail RWG 116 may be formed as part of layer 108. RWG 116 may includea top surface and two side surfaces. Each side surface may form an anglewith another surface of layer 108 relative to active region 106 to forma ridge of RWG 116. Each angle may be less than 90°.

FIG. 2 shows cross section of a semiconductor laser 200. Laser 200 issimilar to laser 100 of FIG. 1 because it similarly includes activeregion 106, and layers 102, 104, 108, and 110. Laser 200 also includesdovetail RWG 116 and opening 118. Laser 200, however, further includes apolymer 202 disposed adjacent to layer 110 for capacitance reduction andplanarization to significantly improve a thermal conductivity path fromthe active region. Polymer 202 may be a photo-sensitive polymer. Laser200 also additionally includes layer 204. Layer 204 may be a metallayer, similar to layer 112. Semiconductor laser 200 may provideexcellent planarization and significant capacitance reduction, as wellas significant thermal conductivity improvements. However, laser 200 mayexhibit changes to threshold current, laser wavelength, and/or a sidemode suppression ratio after thermal processing such as metallizationannealing and soldering. These performance changes may inhibit use ofpolymer 202.

FIG. 3 shows a cross-section of semiconductor laser 300. Laser 300 issimilar to lasers 100 and 200 in that it includes active region 106, andlayers 102, 104, and 108. Laser 300 also includes dovetail RWG 116 andopening 118. Laser 300, however, further includes a material 302 andmetal layer 304. Laser 300 may be planarized using a lift-off processfor the deposition of material 302 on layer 108 and dovetail RWG 116such that opening 118 remains. Material 302 may be formed from MgO,MgF₂, SiO₂, Si₃N₄, and/or other non-electrically conducting materialswith low relative dielectric constant (for example, below 10) in afrequency range up to 50 GHz, or in the 10-50 GHz frequency range, forexample. The dielectric constant may be less than 10 in order to reducecapacitance, and may be closer to 1.

Using the planarization as discussed above may provide cooling of thesemiconductor laser active region using material 302. Material 302 maybe evaporated onto laser 300, and may form an evaporated thin film onlaser 300. The evaporation may provide adherence to a variety ofsurfaces, such as semiconductors, insulators, and/or metals, and mayfurther provide adherence to a variety of topologies and shapes formedby such surfaces, such as a dove tailed ridge. The evaporation mayfurther allow for targeted deposition. Metal 304 may be similar to layer204 discussed above.

MgO can be used as an illustrative example of the low relativedielectric material 302 that may be used for planarization. MgO canadhere to a variety of surfaces and topography using evaporation. Forexample, electron beam evaporation of MgO onto a heated substrate oflaser 300 (such as layer 108, for example) that is patterned may allowfor deposition to occur in targeted areas. MgO has a thermalconductivity of 43 W/mK which may enable heat conduction from activeregion 106 of laser 300.

Further, MgO can also be used to planarize a surface of laser 300allowing for less complicated metallization techniques which haveimproved characteristics. For example, current photosensitiveplanarization polymers may have poor thermal conductance. Utilizing thedovetail ridges for low electrical resistance (and large contact area)of laser 300, as well as electron beam or thermal evaporation of MgO onlaser 300, may allow for a robust low cost fabrication process. Theplanarization described may provide a durable technique in reducing thesemiconductor laser device temperature that can also be applied to othersemiconductor devices.

MgO may also include a complex refractive index that is appropriate forthe semiconductor laser structure since it may have a reduced absorptionat the laser wavelength. Here, the imaginary component of refractiveindex may be very low and the real component of refractive index may beappropriate for allowing appropriate waveguiding for laser 300.

One important advantage of the planarization of the present disclosureis that it may improve the path for dissipating the heat from and activeregion (such as active region 106) of a semiconductor laser. Heat may begenerated in a dovetail RWG (e.g., RWG 116) by resistive or Jouleheating, and heat may also be generated in the active region. This heatcan be conducted away from the laser through the substrate (which may,for example, be made from InP), laterally through a dielectric (forexample, MgO), or through a metal contact on the top of the RWG.

FIG. 1 shows a structure where the metal layer 112 is deposited so as towrap around the sides of the RWG 116. Deposition of the metal, even in aplanetary or rocking fixture, may lead to a thinner layer of metal layer112 on the ridge sidewalls of RWG 116 as the sidewalls may never benormally facing to the metal deposition source. The thinner layer ofmetal layer 112 may present a restricted thermal path from the top ofRWG 116 to a bond pad 114 in FIG. 1. FIG. 3 shows a structure where theridge areas to each side of RWG 116 have been filled with a dielectric(e.g., material 302). This planarizes the structure and allows metal 304to be deposited at near to normal incidence. The metal 304 may bedeposited as a thicker flat layer that may offer a better path for heatenergy to reach the bond pad area. An air bridge has also been used toplanarize the structure, and will be discussed in relation to FIG. 4.

FIG. 4 shows a cross-section of semiconductor laser 400. Semiconductorlaser 400 is similar to laser 300 in that it includes active region 106,and layers 102, 104, and 108. Laser 400 also includes dovetail RWG 116,opening 118, layer 110, and metal 304. Laser 400, however, also includesair gaps 402. Metal 304 acts as a metal air-bridge over air gaps 402,and is configured to connect a bond pad to the top of laser 400. Laser400 is formed by removing a polymer (formed from polymer 202, forexample) from either side of dovetail RWG 116 to form air gaps 402. Thepolymer may be removed using solvents after metal 304 has been depositedon the structure, for example. Part or all of the polymer may be removedas follows.

A first layer of resist may be planarized onto the structure andpatterned such that it fills each side of dovetail RWG 116. The firstlayer of resist may then be cured such that, for example, it is notsoluble in a developer, but may still be soluble in acetone and otherresist stripping agents. A second layer of resist may then be patternedon top of the first layer to form a lift-off structure. After metalevaporation of metal 304 on the structure, the layers of resist may beremoved, removing the polymer and leaving air gaps 402 between layer 110and metal 304. This design may reduce unstable laser operation and mayprovide a thick planarized metal film for heat conduction.

FIG. 5 shows top side views of adjustable fixture 502. A semiconductorlaser may be attached to fixture 502 via a wafer plate, and evaporationmay be performed to turn the laser into a semiconductor laser 300. Whenthe laser is attached to fixture 502, fixture 502 may be positioned suchthat electron-beam evaporation of material 302 (such as MgO, forexample) onto right and left sides of dovetail RWG 116 of the laser andunderneath dovetail RWG 116 may be performed. As indicated by FIG. 5,fixture 502 may be used for evaporation on the right side ridges ofdovetail RWG 116, and may provide for such evaporation at a time T1.Fixture 502 may be repositioned, thereby repositioning the laser, andused for evaporation on the left side ridges of dovetail RWG 116 on thelaser at a time T2. Laser 300 may thereby be formed.

Fixture 502 may include an integrated heater that may adjust evaporationtemperature for improved evaporated thin-film properties. Theflexibility of the setup provided by fixture 502 may allow for theaccommodation of a wide range of angled dovetail structures. FIGS. 6Aand 6B show different configurations of fixture 502. FIG. 6A showsfixture 502 with heater 602. Fixture 502 supports a wafer 604 and holdsit in place for evaporation of a material, such as material 302 viaevaporation stream 606, onto it. Wafer 604 may be any kind ofsemiconductor device. As discussed above, heater 602 may be used toadjust evaporation temperature when an evaporant stream 606 of material302 is directed toward wafer 604. Fixture 502 may be adjusted todifferent angles such that evaporant stream 606 can be applied todifferent areas of wafer 604. Moreover, the rate at which evaporationstream 606 is applied to wafer 604 may be adjustable.

FIG. 6B shows fixture 502 configured to support a laser 608. Heater 602is not shown in FIG. 6B, but may be integrated into fixture 502.Alternatively, heater 602 may be an external unit as shown in FIG. 6A.Laser 608 include the same elements as laser 300, except material 302evaporated onto it. Via evaporant stream 606, material 302 may beevaporated onto laser 608 to form laser 300. For example, fixture 502may be angled such that laser 608 and dovetail RWG 116 is orientedtoward evaporant stream 606. Evaporant stream 606 may therefore beevaporated onto laser 608 and on a surface of RWG 116. As discussedabove, material 302 may be MgO. Therefore, MgO may be deposited ontolaser 608 via evaporant stream 606. Fixture 502 may be positioned at anangle of 45° as shown in FIG. 6B. Fixture 502, however, may beadjustable such that it forms an angle ranging from 0° to 360°. Also,heater 602 can be varied from ambient temperature to 300° C., forexample. For example, the temperature during evaporation may be 105° C.Changes in substrate temperature during evaporation may impactevaporated film density on laser 608 and optical/thermal properties oflaser 608. Fixture 502 therefore provides for evaporation via electronbeam evaporation at low substrate temperatures and control ofevaporation angle and wafer temperature, and thereby providesplanarization of devices, such as laser 608.

Using fixture 502, the growth of highly thermal conductive material onthe ridge sidewalls of dovetail RWGs using compatible and low costtechniques like electron beam evaporation or sputtering techniques maybe performed. Tunable semiconductor devices may therefore be produced.

Planarization using material 302 can be performed, where material 302may be MgO, MgF₂, SiO₂, Si₃N₄, and/or other dielectrics. Thermalconductivity may be slightly improved using MgO and MgF₂ compared toSiO₂. Such planarized structures may exhibit significant capacitancereduction and an improved thermal conductive path from the activeregion. Unlike the case of a polymer, device characteristics may remainstable after significant thermal processing, such as metallizationannealing and soldering. Also, mechanical strength of the dovetail ridgemay be improved.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Further, although the present disclosure hasbeen described herein in the context of at least one particularimplementation in at least one particular environment for at least oneparticular purpose, those of ordinary skill in the art will recognizethat its usefulness is not limited thereto and that the presentdisclosure may be beneficially implemented in any number of environmentsfor any number of purposes.

1. A laser structure, comprising: a substrate; an active region arrangedon the substrate; a waveguide arranged on the active region, thewaveguide including a first surface and a second surface that join toform a first angle relative to the active region; and a materialdeposited on the first surface and the second surface of the waveguide.2. The laser structure of claim 1, wherein the first angle is less thanninety degrees.
 3. The laser structure of claim 1, wherein the waveguidefurther includes a third surface and a fourth surface that join to forma second angle relative to the active region, and the material isdeposited on the third surface and the fourth surface.
 4. The laserstructure of claim 3, wherein the second angle is less than ninetydegrees.
 5. The laser structure of claim 1, wherein the material is oneof MgO, MgF₂, SiO₂, or Si₃N₄.
 6. The laser structure of claim 1, whereinthe material has a dielectric constant below 10 in a frequency range upto 50 GHz.
 7. The laser structure of claim 1, wherein the material isnon-conducting.
 8. The laser structure of claim 1, wherein the waveguideincludes a fifth surface arranged between the first surface and thethird surface, and the laser structure further comprises: a firstcontact arranged on the fifth surface; and a second contact arranged onthe substrate, wherein the first contact is configured to bias the laserstructure by delivering electrical current to the laser structure. 9.The laser structure of claim 1, further comprising at least one facet.10. The laser structure of claim 9, wherein the at least one facet isformed in the active region.
 11. A laser structure, comprising: asubstrate; an active region arranged on the substrate; a waveguidearranged on the active region, the waveguide including a first ridge anda second ridge, wherein the first ridge forms a first air gap and thesecond ridge forms a second air gap; and a metal layer arranged as abridge over the first and second air gaps.
 12. The laser structure ofclaim 11, wherein the first ridge forms a first angle of less thanninety degrees relative to the active region and the second ridge formsa second angle of less than ninety degrees relative to the activeregion.
 13. The laser structure of claim 11, wherein a first contact isarranged on a surface of the waveguide and the first contact isconfigured to bias the laser structure by delivering electrical currentto the laser structure.
 14. The laser structure of claim 11, furthercomprising at least one facet.
 15. The laser structure of claim 14,wherein the at least one facet is formed in the active region.
 16. Amethod of fabricating a laser structure, comprising: arranging an activeregion on a substrate; arranging a waveguide on the active region, thewaveguide including a first ridge and a second ridge; depositing apolymer on the waveguide such that the polymer coats underneath thefirst ridge and second ridge; depositing at least one layer of resist onthe polymer; depositing a metal layer on the at least one layer ofresist; and removing the deposited polymer and the deposited at leastone layer of resist.
 17. The method of claim 16, wherein the first ridgeforms a first angle of less than ninety degrees relative to the activeregion and the second ridge forms a second angle of less than ninetydegrees relative to the active region.
 18. The method of claim 16,further comprising arranging a first contact on a surface of thewaveguide, wherein the first contact is configured to bias the laserstructure by delivering electrical current to the laser structure. 19.The method of claim 16, further comprising forming at least one facet inthe active region.
 20. The method of claim 19, wherein the at least onefacet is formed by etching.
 21. A fixture for electron beam evaporation,the fixture comprising: a wafer plate configured to support a laserstructure; and an integrated heater configured to emit heat at anevaporation temperature, wherein the heat emitted by the integratedheater is adjustable and an angle of the fixture is adjustable.
 22. Thefixture of claim 21, wherein the wafer plate is further configured toorient the laser structure relative to an evaporant.
 23. The fixture ofclaim 21, wherein the orientation of the laser structure subjects atleast one ridge of the laser structure to the evaporant.
 24. The fixtureof claim 21, wherein the orientation of the laser structure is variedbased on an angle of at least one ridge of the laser structure.